Automated warehousing systems have been known in the art for many years. Typically, an automated warehousing system comprises a carriage which delivers bins to, or retrieves bins, from one or more storage racks. Each storage rack has an array of horizontally and vertically oriented bin locations (stations). Each bin location is designed to support a bin filled with materials. Each bin location has a different numerical designation, representing the horizontal and vertical coordinates of the bin location in the storage rack.
A computer controls movement of the carriage relative to the storage rack, and the bin locations thereon. The computer causes the carriage, and a load-handling device supported on the carriage, to move to a selected bin location and to either deliver a bin to the selected bin location or retrieve a bin from the selected bin location. A bin retrieved from a selected bin location is delivered either to a pick-up and delivery station or to another bin location. Moreover, a bin can be retrieved from the pick-up and delivery station and delivered to a selected bin location.
When the system is initially turned on, it is in what is referred to in the art as a "power-up" mode and must find an initial point of reference. When the system is in a "power-up" mode, the carriage may be in any particular location, depending upon the last task the carriage was directed to perform before power to the system was discontinued. At "power-up", it is important that the system establish a floating, zero offset, reference point from which subsequent movement of the carriage can be controlled. One known way of establishing such a floating, zero offset, reference point is by means of a reed type proximity switch sensor and a magnet pair. The magnet is gapped from the reed at a distance which exceeds the maximum limits of yaw of the carriage about its center of travel. The floating, zero offset, reference point is established by passing a magnetic shield (in the form of a vane) into and then out of the gap between the reed and magnet. This causes the reed switch to open and then close, to establish the floating, zero offset, reference point for the system. Thus, at "power-up", the carriage will move in a direction which causes the vane to pass through the gap, to establish the floating, zero offset, reference point. The system then uses that floating, zero offset, reference point to control subsequent movements of the carriage.
One problem with such types of magnetic proximity sensors is that in time and under the operating conditions of the system, the location of the floating, zero offset, reference point may vary. This may be due to changes in magnet field strength, temperature, vibration and other electrical and metallurgical reasons effecting the opening and/or closure of the switch contacts. Moreover, over time dirt and contaminate built up on the elements of the magnetic proximity sensor switch and gap can affect the sensing of the floating, zero offset, reference point of the system. As the floating, zero offset, reference point of the system varies, the entire system may thus lose the degree of precision required to properly control the carriage and the load handling device on the carriage.